Abstract : In this document we study mirror surface specifications for the Advanced Virgo Output Mode Cleaner (OMC). It is well known that flatness defects and micro-roughness on mirror surfaces affect the resonant mode in a cavity. Here we would like to determine the mirror surface specifications in order to have more than 99 % of transmission through the OMC for a perfect Gaussian input beam TEM00. To do this determination we will simulate the cavity with the code OSCAR (Degallaix 2010). Two kinds of losses will be characterised. The first one is the part of resonant beam which go out of mirrors, such losses are called sometimes "diffraction losses" or "clipping losses". The second, which can be the most important loss, is the reflection of the incident beam which does not match the imperfect cavity. More details on http://lucdigallo.free.fr/spip.php?article34&lang=en